posix: sysconf.c uses POSIX values

We need to add _POSIX_C_SOURCE to this file as it uses POSIX values from
limits.h including IOV_MAX, CHILD_MAX and ARG_MAX.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

lib/posix/options/sysconf.c

@@ -4,6 +4,8 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#undef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
 #include <zephyr/posix/pthread.h>
 #include <zephyr/posix/sys/sysconf.h>
 #include <zephyr/posix/unistd.h>